Gigabit service running at less than 50mb

I repeated call xfinity support because internet goes back to 1995 speeds even though I have gigabit service. A good support rep will get wireless up to 500MB. Then I come home a few days later and speed is back to 50MB or less. I'm tired of calling support. It seems like xfinity fixes it momentarily then dials it back until you complain. I'm hoping SpaceX internet comes up soon. Anyone else having this problem?